The present invention relates to microwave circuits and more particularly to means for suppressing ridge waveguide modes that can propagate in such circuits.
Microwave circuits, such as a strip line low pass filter, are typically placed on top of a ground plane block and then both are mounted within a channel. Ideally, the width of the block should be the same as that of the channel. However, due to the requirements of being able to make design changes, or not being able to machine the block to the exact width at a reasonable cost, it is not always feasible to make both widths exactly the same. Therefore, spaces occur between the block and the box, which spaces form, with other surfaces present, a "ridge waveguide." This type of waveguide has a much lower cutoff frequency than that of an ordinary one of similar overall external dimensions. If the microwave circuit is a low pass filter, the result can be that signals having a frequency that are higher than that of the filters cutoff frequency and lower than the cutoff frequency of the waveguide formed by the channel in the box are propagated along the ridge guide, thus bypassing the filter. This is an effective raising of the filter's cutoff frequency which is not desired.